Method and apparatus for a linearized output driver and terminator

ABSTRACT

A method and apparatus for a linearized output driver and terminator is described. In one embodiment the method includes forming a gate electrode on a substrate, the portion of the substrate covered by the gate electrode defining a channel. The method further includes forming a first source/drain doped region on laterally opposed sides of the gate electrode in the substrate. The method also includes forming a spacer on laterally opposed sides of the gate electrode on the substrate. The method also includes forming a linearized drain contact region at a location within the first source/drain doped region sufficiently distant from the gate electrode to define a series resistor in the first source/drain doped region disposed between the gate electrode and the linearized drain contact area based on an expected resistivity of the source/drain doped region, the series resistor coupled electrically to the channel.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is related to the field of semiconductor circuit design and processing and more particularly to design of buffers suitable for use with transmission line conductors associated with integrated circuits.

[0003] 2. Description of the Related Art

[0004] In many environments in which semiconductor devices are used, the conductors used to link the various devices behave in a manner similar to a transmission line. FIG. 1A illustrates a prior art transmission line environment. Buffer 110 drives transmission line 120, which is terminated to ground by termination impedance 130. Termination impedance 130 and buffer 110 preferably have the same impedance Z₀. With such a matched impedance situation, reflections are minimized leading to enhanced signal quality.

[0005] It is common to use resistive termination for buses on printed circuit boards, and many semiconductor devices are mounted in packaged form on printed circuit boards. Resistors have a well-known voltage transfer curve (IV curve). Ideally, a resistor has a linear voltage transfer curve, and a typical resistor may be assumed to have an ideal or linear voltage transfer curve. However, the buffers contained within semiconductor devices (such as buffer 110) rarely have a linear voltage transfer curve.

[0006]FIG. 1B illustrates a voltage transfer curve plot for various circuit elements. In particular, the straight line marked R_(ideal) represents the linear voltage transfer curve of a resistor and the curved lines represent voltage transfer curves for an insulated gate FET device. Moreover, the dashed lines R_(ac1) and R_(ac2) represent the slope of the IV curve for the insulated gate FET at two regions, and it will be apparent that the difference in slope between R_(ac1) and R_(ac2) is significant. An insulated gate FET biased in a certain operational area may be expected to behave as a resistor for small deviations from the bias point (small signals). R_(ac1) and R_(ac2) may represent the resistance of an insulated gate FET for small signals when biased at the points where the corresponding IV curve of the insulated gate FET intersects those lines. The nonlinear MOS IV characteristics can result in imperfect termination, thereby increasing inter-symbol interference and degrading signal quality.

[0007] It is recognized that a combination of the insulated gate FET and an additional resistor (such as an off-chip series resistor for example) may achieve a more linear IV characteristic. However, this requires an additional component and corresponding opportunities for yield loss or variation in the circuitry. Similarly, a resistor may be formed on-chip between the bond pad and the insulated gate FET structure (or between the insulated gate FET structure and a power/ground rail). However, this may also require additional processing steps, may use up area that must be dedicated at the edge of the device, and may result in additional constraints on the overall design of the device.

SUMMARY OF THE INVENTION

[0008] A method and apparatus for a linearized output driver and terminator is described. In one embodiment the method includes forming a gate electrode on a substrate, the portion of the substrate covered by the gate electrode defining a channel. The method further includes forming a first source/drain doped region on laterally opposed sides of the gate electrode in the substrate. The method also includes forming a spacer on laterally opposed sides of the gate electrode on the substrate. The method also includes forming a linearized drain contact region at a location within the first source/drain doped region sufficiently distant from the gate electrode to define a series resistor in the first source/drain doped region disposed between the gate electrode and the linearized drain contact area based on an expected resistivity of the source/drain doped region, the series resistor coupled electrically to the channel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention is illustrated by way of example and not limitation in the accompanying figures.

[0010]FIG. 1A illustrates a prior art transmission line environment.

[0011]FIG. 1B illustrates a voltage-current plot for various circuit elements.

[0012]FIG. 2 illustrates an embodiment of an insulated gate FET.

[0013]FIG. 3 illustrates a schematic of the insulated gate FET of FIG. 2.

[0014]FIG. 4A illustrates an alternate embodiment of an insulated gate FET.

[0015]FIG. 4B illustrates a schematic of the insulated gate FET of FIG. 4A.

[0016]FIG. 5A illustrates an early stage of formation of an embodiment of an insulated gate FET.

[0017]FIG. 5B illustrates an intermediate stage of formation of an embodiment of an insulated gate FET.

[0018]FIG. 5C illustrates a later stage of formation of an embodiment of an insulated gate FET.

[0019]FIG. 6A illustrates an early stage of formation of an alternate embodiment of an insulated gate FET.

[0020]FIG. 6B illustrates an intermediate stage of formation of an alternate embodiment of an insulated gate FET.

[0021]FIG. 6C illustrates a later stage of formation of an alternate embodiment of an insulated gate FET.

[0022]FIG. 7A illustrates an embodiment of a process of forming an insulated gate FET.

[0023]FIG. 7B illustrates an alternate embodiment of a process of forming an insulated gate FET.

DETAILED DESCRIPTION

[0024] A method and apparatus for a linearized output driver and terminator is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention can be practiced without these specific details. In other instances, structures and devices are shown in block diagram form in order to avoid obscuring the invention.

[0025] Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. The appearances of the phrase “in one embodiment” in various places in the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments.

[0026] Throughout the description, reference is made to structures which are formed, and this may mean a variety of things to one skilled in the art. For example, an implant layer may be formed through diffusion or ion implantation for example. Oxides may be formed through CVD or other methods. Additionally, the description refers to insulated gate FETs (Field Effect Transistors), a common embodiment of which is a MOSFET.

[0027] A method and apparatus are presented in which an insulated gate FET suitable for use in a transmission line environment and formation of such an insulated gate FET is illustrated. The insulated gate FET has formed within it a series resistor which linearizes the voltage transfer curve of the insulated gate FET. Furthermore, the series resistor is formed during the normal processing of the integrated circuit containing the insulated gate FET, thus reducing the potential for additional yield loss or process complexity due to the presence of the resistor. Additionally, the series resistor may be expected to occupy a relatively small area and have a reasonably predictable resistance for layout and circuit design purposes.

[0028]FIG. 2 illustrates an embodiment of an insulated gate FET. Substrate 300 may be a semiconductor or a doped portion of a semiconductor. Formed on substrate 300 is a gate including a gate dielectric layer 320 and a conductive layer 310. Gate dielectric layer 320 may be an oxide of the substrate for example. Formed on one side of the gate is source doped region 335, while formed on the other side of the gate is resistor/drain doped region 330. In one embodiment these doped regions are lightly doped portions of the substrate 300 which are referred to as the source/drain extension.

[0029] Formed around the gate and above a portion of the resistor/drain doped region 330 and above source doped region 335 are spacers 340. Spacers 340, in one embodiment, are formed of an oxide of the substrate 300. Part of resistor/drain doped region 330 is not covered by a spacer 340, and formed on top of this portion is insulator 350, which in one embodiment is formed from the same oxide which forms part of the spacers 340. In one embodiment, the spacers 340 are formed on two sides of the gate out of a combination of an oxide and nitride of silicon. However, it will be appreciated that many materials may be used to form spacer 340, though it is preferable that those materials have an electrically insulating character. Spacers 340 are often included to satisfy design rules concerning the placement of various features on a semiconductor, such as hot electron robustness of the gate.

[0030] Formed on one side of doped region 335 and the opposite side of doped region 330 are contact regions. Contact region 365 is a contact for a drain of the insulated gate FET, and contact region 360 is a source contact of the insulated gate FET. In one embodiment, contact regions 360 and 365 are heavily doped portions of substrate 300, and they may be formed for the purpose of achieving an ohmic contact with a conductor.

[0031]FIG. 3 illustrates a schematic of the insulated gate FET of FIG. 2. Insulated gate FET 370 is the ideal insulated gate FET, while R_(s) 390 represents the area of doped region 335 and R_(LINEAR) 380 represents the area of doped region 330. R_(LINEAR) 380 may be controlled during formation such that it is resistive enough to provide a convenient resistor for the output stage of a buffer used to drive a transmission line. The following table illustrates the characteristics of several different types of resistors that may be formed in a semiconductor device and their properties. Rela- tive Range of Accu- Temp. Voltage Absolute Values racy Coeff. Coeff. Accuracy Resistor Type (Ω/sq) (%) (ppm/V) (ppm/V) (%) Diffused  10-100 2% 1500 200 35% (Unsilicided) Diffused   2-3 (Silicided) Poly  30-200 2% 1500 100 30% (Unsilicided) Poly   2-3 (Silicided) P-Well   1-10000 2% 8000 10000 40% Pinch   5-20000 10% 10000 20000 50% Ion Implanted 0.5-2000 1% 400 800 5%

[0032] As will be appreciated, the last entry represents the resistance formed through ion implantation or diffusion such as that used to form the resistor/drain doped region 330. This type of resistor exhibits a balance of the various properties illustrated in the table, and thus may be expected to form a fairly predictable and repeatable resistance during manufacture of a semiconductor.

[0033]FIG. 4A illustrates another alternate embodiment of an insulated gate FET. Substrate 400 may also be a semiconductor or a doped portion of a semiconductor. Formed on substrate 400 is a gate including a gate dielectric layer 410 and a conductive layer 420. Formed on either side of the gate are source/drain doped regions 430, while formed on the drain side of the gate is resistor doped region 435. In one embodiment each of these doped regions (430 and 435) are lightly doped portions of the substrate 300.

[0034] Formed around the gate and above the source/drain doped regions 430 are spacers 440. Resistor doped region 435 is not covered by a spacer 440. Formed on top of doped region 435 is insulator 465, which in one embodiment is formed from the same oxide which forms part of the spacers 440. Formed on the source side of the insulated gate FET adjacent to doped region 430 is contact 450 which may be used as a source contact. Formed between source/drain doped region 430 and resistor doped region 435 is contact 455 which may be used as a drain contact. Formed beyond and adjacent to resistor doped region 435 is contact 460, which may be used as a linearized drain contact.

[0035]FIG. 4B illustrates a schematic of the insulated gate FET of FIG. 4A. Insulated gate FET 470 is the ideal insulated gate FET, while R_(S) 480 represents the source area of source/drain doped regions 430. R_(LDD) 490 represents the drain area of source/drain doped regions 430. R_(S) 480 and R_(LDD) 490 may be resistive enough that they can be modeled as part of the insulated gate FET. R_(LINEAR) 495 represents the area of resistor doped region 435. R_(LINEAR) 495 may be controlled during formation by properly placing the contact 460 (linearized drain contact) relative to the contact 455 (drain contact) such that it is resistive enough to provide a convenient resistor for the output stage of a buffer used to drive a transmission line. As a result, the node between R_(LDD) 490 and R_(LINEAR) 495 may be used as an unlinearized drain contact to the device, while the unconnected end of R_(LINEAR) 495 may be used as a linearized drain contact.

[0036] Furthermore, the layout illustrated in FIG. 4A may cure registration errors or misalignment when forming the resistor. The resistor is formed along with the formation of the source/drain and contact regions in common lithographic steps, rather than requiring additional lithographic steps and thus increasing the complexity of the process. Additionally, it will be appreciated that in general the layouts illustrated may be used for either NMOS or PMOS devices, they are not dependent on a particular type of dopant.

[0037]FIG. 5A illustrates an early stage of formation of an embodiment of an insulated gate FET. Substrate 500 may be a semiconductor or a doped portion of a semiconductor. Formed on substrate 500 is a gate including a gate dielectric layer 510 and a conductive layer 520. In one embodiment, a thin oxide layer (of the substrate) is formed over the entire substrate 500 and a conductive layer is formed over the thin oxide layer. Then, both the conductive layer and the oxide layer are selectively etched to form the gate dielectric layer region 510 and conductive layer region 520. Formed on either side of the gate is source/drain doped region 530. In one embodiment these doped regions are lightly doped portions of the substrate 500.

[0038]FIG. 5B illustrates an intermediate stage of formation of an embodiment of an insulated gate FET. The device as illustrated in FIG. 5A next has an insulating layer 540 formed on its surface. In one embodiment, this is a TEOS layer, formed through a CVD process, and it either is prevented from forming on the gate region or is selectively etched from the gate region. Note that the insulating layer 540 may be different in properties and thickness from the gate dielectric layer 510. Formed on top of insulating layer 540 is a second insulating layer which is then selectively etched to form the two spacers 550. Note that the overall spacer may actually be a combination of insulating layer 540 and spacer portions 550. In one embodiment, the spacers 550 are formed of a nitride of silicon and the insulating layer 540 is formed of an oxide of the substrate.

[0039]FIG. 5C illustrates a later stage of formation of an embodiment of an insulated gate FET. Portions of insulating layer 540 are etched selectively, leaving insulating layer portions 540A, 540B, and 540C (a surrounding layer in one embodiment). Next, contact region doped regions are formed for the source and drain (effectively forming the source and drain), resulting in the formation of source contact region 560A and drain contact region 560B. Note that these contact regions disrupt the uniformity of the original source/drain doped region 530, resulting in the formation of source doped region 530A and drain doped region 530B. After the contact region doped regions are formed, a conductive material such as Titanium is deposited above the contact regions and then heated, resulting in the formation of source metal silicide 570A and drain metal silicide 570B. The resulting structure forms one embodiment of a linearized insulated gate FET structure. Other materials which may be used to form a silicide include but are not limited to Tungsten, Molybdenum, Cobalt, and Nickel.

[0040]FIG. 6A illustrates an early stage of formation of an alternate embodiment of an insulated gate FET. Substrate 600 may be a semiconductor or a doped portion of a semiconductor. Formed on substrate 600 is a gate including a gate dielectric layer 610 and a conductive layer 620. Formed on one side of the gate is source doped region 630S and on the other side is drain doped region 630D. In one embodiment these doped regions (630S and 630D) are lightly doped portions of the substrate 600.

[0041]FIG. 6B illustrates an intermediate stage of formation of an alternate embodiment of an insulated gate FET. The device as illustrated in FIG. 6A next has an insulating layer (such as an oxide of the substrate) formed on its surface resulting in insulating layers 640S and 640D. Formed on top of the insulating layer is a second insulating layer (such as a nitride of silicon) which is then selectively etched to form the two spacers 650S and 650D. Note that the overall spacer may actually be a combination of the insulating layer and the second insulating layer spacer portion.

[0042]FIG. 6C illustrates a later stage of formation of an alternate embodiment of an insulated gate FET. Portions of the insulating layer are etched selectively, leaving insulating layer portions 640S and 640D. Next, contact region doped regions are formed, resulting in the formation of source contact region 660S, drain contact region 660D and linearized drain contact region 660L. Note that these contact regions disrupt the uniformity of the original source/drain doped regions 630S and 630D, resulting in the formation of the illustrated source extension area 630S and drain extension areas 630D. Moreover, note that the drain extension area 630D disposed between the regions 660D and 660L forms the resistor which results in the linearized drain property. After the contact region doped regions are formed, a conductive material such as Titanium is formed above the contact regions and heated, resulting in the formation of source metal silicide 670S, drain metal silicide 670D and linearized drain metal silicide 670L. The resulting structure forms an alternate embodiment of a linearized insulated gate FET structure.

[0043] Note that the resistivity of the various portions of the substrate is fairly predictable, so the location at which to form various regions such as the linearized contact region 660L for example, may be determined based on the expected resistivity of the doped region between the linearized drain contact region 660L and the drain contact region 660D and the desired resistance of the resulting series resistor. It will be appreciated that process variations may result in corresponding variations in the value of the resistor, and that determinations about where to form the contact region or other region tend to be made during design and layout of the circuit rather than during actual fabrication of the circuit.

[0044]FIG. 7A illustrates an embodiment of a process of forming an insulated gate FET. It will be appreciated that the processes illustrated in FIGS. 7A and 7B are illustrative and that one skilled in the art will understand that these illustrations pertain to portions of the process without detailing the implementation of many well-known techniques utilized in performance of these processes. At block 710 a gate is provided on a semiconductor substrate, such as forming and selectively etching an oxide and a conductor. At block 720 a LDD source/drain implant is performed. At block 730, a spacer is formed, such as a spacer made of a combination of two insulating layers such as Si₃N₄ and SiO₂.

[0045] At block 770 source/drain contact processing is performed. In one embodiment, this includes selectively etching the insulator(s) used to form the spacer and doping the actual source and drain regions to a high dopant concentration, while utilizing photoresist or a blocking material of some form to shield the rest of the semiconductor from the doping. Typically, the regions thus doped will principally serve as areas where contact with conductors may be made as discussed below. In relation to the earlier described layouts, this corresponds to the first part of formation of the contact regions, in which areas which may either be thought of as the actual source and drain or the contact portions of the source and drain are formed. Positioning of these source and drain regions results in definition of the area which serves as a series resistor coupled to the drain of the insulated gate FET. This positioning is designed with an understanding of what the resistivity should be for the area which will serve as the series resistor, often based on models of the process or results from use of the process prior to design activities. If the source and drain regions thus formed did not leave a sufficient lightly doped region, the high dopant concentration of the source and drain regions would result in a region nearly as conductive as polysilicon where the resistor was supposed to be formed.

[0046] Finally, at block 780, metallization is performed, in which metal is deposited in the source/drain contact regions to form conductive contact with the substrate. Metallization may involve either deposition (such as evaporation or CVD) of the metal on a photoresist pattern or deposition of the metal followed by deposition of a photoresist pattern and then an appropriate etch. Furthermore, metallization typically results in formation of a silicide bond at the junction between the deposited metal and the substrate, as the entire workpiece is heated to a sufficient temperature to cause silicidation to occur.

[0047]FIG. 7B illustrates an alternate embodiment of a process of forming an insulated gate FET. Formation of the spacer is further illustrated as formation of a first insulator such as a TEOS layer at block 740. The spacer formation further includes formation of a second insulation layer such as a Si₃N₄ layer at block 750. Finally, the spacer formation includes etching the second insulation layer such as etching a Si₃N₄ layer at block 760.

[0048] In the foregoing detailed description, the method and apparatus of the present invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. For example, the apparatus of FIGS. 2 and 4A may be modified to conform to processing requirements while still fitting within the spirit and scope of the invention. Likewise, the processes of FIGS. 7A and 7B may be modified or may include many additional components or may include portions of the process performed in parallel within the spirit and scope of the invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive. 

What is claimed is:
 1. A method comprising: forming a gate electrode on a substrate, the portion of the substrate covered by the gate electrode defining a channel; forming a first source/drain doped region on laterally opposed sides of the gate electrode in the substrate; forming a spacer on laterally opposed sides of the gate electrode on the substrate; forming a linearized drain contact region at a location within the first source/drain doped region sufficiently distant from the gate electrode to define a series resistor in the first source/drain doped region disposed between the gate electrode and the linearized drain contact region based on an expected resistivity of the source/drain doped region, the series resistor coupled electrically to the channel.
 2. The method of claim 1 further comprising: forming a source contact area within the first source/drain doped region at a location adjacent to the spacer and on an opposite side of the gate electrode from the linearized drain contact area, the source contact area formed simultaneously with the forming of the linearized drain contact area.
 3. The method of claim 2 further comprising: forming a drain contact area within the first source/drain doped region at a location adjacent to the spacer and disposed between the gate electrode and the linearized drain contact area, the drain contact area formed simultaneously with the forming of the linearized drain contact area.
 4. The method of claim 3 wherein: forming a first contact hole through a layer disposed above the substrate to the substrate at a location within the linearized drain contact region; providing a second contact hole through the layer disposed above the substrate to the substrate at a location within the source contact area; providing a third contact hole through the layer disposed above the substrate to the substrate at a location within the drain contact area; and each contact hole formed by selectively etching the layer disposed above the substrate.
 5. The method of claim 1 wherein: forming the spacer includes: depositing a first insulating material on the substrate to form the layer disposed above the substrate; depositing a second insulating material on the layer disposed above the substrate; and selectively etching the second insulating material.
 6. The method of claim 1 further comprising: forming the source/drain doped region by implanting ions into the substrate.
 7. The method of claim 1 further comprising: forming the source/drain doped region by diffusing ions into the substrate.
 8. The method of claim 1 wherein: forming the gate electrode includes: depositing a gate oxide; depositing a gate conductor; selectively etching the gate conductor and the gate oxide.
 9. The method of claim 1 wherein: the first insulating material being an oxide of the substrate and the second insulating material being silicon nitride.
 10. The method of claim 2 further comprising: forming a conductor on the substrate through the first and second contact holes; and forming a silicide of the conductor and the substrate.
 11. The method of claim 8 wherein: the substrate being silicon, the first insulating material being silicon oxide, and the gate conductor being poly-silicon.
 12. An apparatus comprising: an insulated gate FET having a gate electrode; the insulated gate FET having a channel disposed beneath the gate electrode; the insulated gate FET having a source doped region adjacent to the gate electrode; the insulated gate FET having a drain doped region adjacent to the gate electrode and laterally opposite the source doped region; the insulated gate FET having a source contact area disposed adjacent to the source doped region away from the gate electrode; and the insulated gate FET having a linearized drain contact area disposed adjacent to the drain doped region away from the gate electrode, the drain contact area located at a distance from the gate electrode suitable for definition of a series resistor of a predetermined resistance in the drain doped region based on an expected resistivity of the drain doped region.
 13. The apparatus of claim 12 wherein: the insulated gate FET further having a drain contact area disposed within the drain doped region, the drain contact area positioned to divide the drain doped region into a resistor doped region away from the gate electrode and a drain area close to the gate electrode, the resistor doped region being the series resistor.
 14. The apparatus of claim 13 wherein: the insulated gate FET further having a first spacer adjacent to the gate electrode and directly above the source doped region and a second spacer adjacent to the gate electrode and directly above the drain doped region.
 15. The apparatus of claim 14 wherein: each spacer including a layer composed of an oxide of the substrate and a layer composed of a nitride of silicon; the gate electrode including a gate dielectric layer disposed above the substrate and a gate conductor disposed above the gate dielectric layer.
 16. The apparatus of claim 15 wherein: the source doped region and the drain doped region are ion-implanted regions.
 17. An apparatus comprising: an insulated gate FET having a gate electrode; the insulated gate FET having a channel disposed beneath the gate electrode; the insulated gate FET having a source doped region adjacent to the channel and electrically coupled to the channel; the insulated gate FET having a drain doped region adjacent to the channel and laterally opposite the source doped region, the drain doped region electrically coupled to the channel; the insulated gate FET having a resistor doped region adjacent to the drain doped region, away from the gate electrode and separated from the drain doped region, the resistor doped region, the drain doped region and the source doped region formed in a common processing step; the insulated gate FET having a source contact area disposed adjacent to the source doped region away from the gate electrode and electrically coupled to the source doped region; the insulated gate FET having a drain contact area disposed between the drain contact area and the resistor doped region and electrically coupled to the drain contact area and to the resistor contact area; and the insulated gate FET having a linearized drain contact area disposed adjacent to the resistor doped region away from the gate electrode and electrically coupled to the resistor doped region.
 18. The apparatus of claim 17 wherein: the insulated gate FET further having a first spacer adjacent to the gate electrode and directly above the source doped region and a second spacer adjacent to the gate electrode and directly above the drain doped region.
 19. The apparatus of claim 18 wherein: the insulated gate FET further having an insulator disposed above the resistor doped region.
 20. The apparatus of claim 18 wherein: each spacer including a layer composed of an oxide of the substrate and a layer composed of a nitride of silicon; the gate electrode including a gate dielectric layer disposed above the substrate and a gate conductor disposed above the gate dielectric layer; and the insulator including a layer composed of an oxide of the substrate and a layer composed of a nitride of silicon.
 21. The apparatus of claim 20 wherein: the source doped region, the drain doped region and the resistor doped region are all ion-implanted regions.
 22. An apparatus comprising: a means for switching; a channel means for selectively conducting, the channel means disposed below and coupled to the means for switching, the channel means conducting in response to the means for switching; a first conductive means for conducting, the first conductive means electrically coupled to the channel means; and a second conductive means for conducting, the second conductive means electrically coupled to the channel means through a series resistor composed of a material, the second conductive means located at a position relative to the channel means to define the series resistor based on an expected resistivity of the material of the series resistor.
 23. The apparatus of claim 22 further comprising: the means for switching comprising an insulator disposed above the channel means and a conductor disposed above the insulator.
 24. The apparatus of claim 23 wherein: the channel means composed of a lightly doped silicon substrate.
 25. The apparatus of claim 24 wherein: the material of the series resistor is an ion-implanted region of a silicon substrate. 